The present invention relates to the deposition of alloy deposits of zinc/manganese alloys from electroplating baths which are at acid pH values close to neutral.
The problem with which the present invention is concerned is to obtain electrodeposits which have high contents of manganese, namely above 9% by weight, but which can be produced without the use of acid ammonium chloride or fluoroborate in the plating bath; these two ingredients being undesirable on environmental grounds.
In addition the process must be able to plate components satisfactorily.
German OLS 2012774 describes a zinc plating process in which the plating bath contains 16.5 g zinc sulphate heptahydrate, 110 g sodium gluconate, 70 g boric acid, 100g anhydrous sodium sulphate, 13 g sodium hydroxide, 0.2 g benzaldehyde and water to make up to one litre, the pH being 6.8. There is no reference to any alloying ingredients being present.
The prior art processes for plating zinc/manganese alloys contain ammonium chloride at acid pH""s. We have attempted to replace the ammonium chloride by alkali metal chloride but found that this did not produce adequate amounts of manganese in the deposits.
Surprisingly we have found that if one uses alkali metal salts with gluconate or tartrate high contents of manganese can be obtained in the electrodeposit.
Thus according to the present invention an electroplating bath for depositing zinc/manganese alloys on a substrate comprises an aqueous bath free or substantially free of ammonium halide and of fluoroborate which is made up from 10-150 g/l alkali metal salt, preferably 25-100 g/l, preferably a sulphate 40-90 g/l boric acid, preferably 50-80 g/l, 10-200 g/l water soluble zinc salt,
preferably 10-100 g/l, more preferably 20-40 g/l, when the alkali metal salt is a halide and
20-200 g/l, preferably 45-100 g/l when the alkali metal salt is a sulphate, 10-50 g/l water soluble manganese salt, preferably 20-40 g/l, 60-140 g/l alkali metal gluconate or tartrate, preferably 110-130 g/l, and alkali metal hydroxide to bring the pH to the range 6.1 to 7.2, preferably 6.1 to 7.0, more preferably 6.3-6.9.
The alkali metal salt can be any such material but the sodium and potassium chlorides or sulphates are the most economical and effective and the sulphates are preferred.
The water soluble zinc salt may be any of those used to electrodeposit zinc but zinc sulphate is preferred.
The water soluble manganese salt may be any of those used to electrodeposit manganese but manganese sulphate, which may be hydrated, is preferred. The zinc and the manganese can be added to the plating bath in the form of salts other than the sulphates for example as sulphamates, methane sulphonates, gluconates, tartrates, acetates, formates, or carbonates. When carbonates are added to acid systems carbon dioxide will be released. This can be a way of avoiding the concentration of the sulphate conductivity salt rising to too high a level. Fairly high concentrations can have benefits in producing more even thickness distribution of the deposit as between high and low current density areas.
Gluconic and tartaric acids are hydroxy carbonic acids, and have been found effective as complexing agents for these systems, however citric acid does not seem to give good results. Other polyhydroxy compounds such as sorbitol might be expected to five stable complexes with zinc, as would amines such as tetra methylene pentamine or EDTA. Triethanolamine does not seem to be able to form a stable complex with zinc in this system.
Additional ingredients which may be added include grain refiners if desired. Water soluble surfactants and polymers are well known in this art for this function and appropriate such materials may be added.
In a preferred form of the invention an electroplating bath is characterised in that it contains benzaldehyde as bisulphite in amount of 50 to 500 mg/l, preferably 100 to 300 mg/l, more preferably 175 to 225 mg/l e.g. about 200 m/l. In another preferred form of the invention an electroplating bath is characterised in that it contains trimethylolpropane in an amount of 1 to 50 g/l, preferably 5 to 25 g/l, more preferably 7.5 to 15 g/l e.g. about 10 g/l.
The bath composition preferably comprises
15-170 g/l of salt anions preferably halide or sulphate anions, preferably 75-140 g/l, more preferably 80-120 g/l,
4-50 g/l zinc ions, preferably 10-18 g/l,
3-16 g/l of manganese ions, preferably 6-13 g/l,
35-90 g/l of borate ions, preferably 60-80 g/l,
50-150 g/l of gluconate or tartrate ions, preferably 80-130 g/l, and preferably 175 to 225 mg/l of benzaldehyde as bisulphite, or 7.5 to 15 g/l of trimethylolpropane.
a pH in the range 6.1 to 7.2, preferably 6.1-7.0, more preferably 6.3-6.9.
One specific embodiment of the invention is the following bath composition
30 g/l zinc chloride, which provides 14.4 g/l of zinc ions and 15.6 g/l of chloride ions,
31 g/l manganese sulphate monohydrate, which provides 10.1 g/l of manganese ions and 17 g/l of sulphate ions,
100 g/l potassium sulphate, which provides 55 g/l of sulphate ions and 45 g/l of potassium ions,
60 g/l boric acid, which provides 57 g/l of borate ions,
120 g/l sodium gluconate, which provides 107 g/l of gluconate ions and 13 g/l of sodium ions,
pH adjusted to 6.5 with sodium or potassium hydroxide.
A preferred specific embodiment of the invention is the following bath composition
65 g/l zinc sulphate heptahydrate, which provides 14.4 g/l of zinc ions and 21.7 g/l of sulphate ions,
30 g/l manganese sulphate monohydrate, which provides 9.8 g/l of manganese ions and 6.5 g/l of sulphate ions,
100 g/l potassium sulphate, which provides 55 g/l of sulphate ions and 45 g/1 of potassium ions,
75 g/l boric acid, which provides 71.3 g/l of borate ions,
120 g/l sodium gluconate or sodium tartrate, which provide 107 g/l of gluconate ions, and 96 g/l of tartrate ions respectively,
pH adjusted to 6.5 with sodium or potassium hydroxide.
Effective plating conditions are room temperature, without agitation, using a zinc anode with a plating current of 2A. However higher or lower temperatures may be used e.g. up to 60xc2x0 C. or down to 10xc2x0 C. Agitation may be used if desired. Plating currents in the range 0.5 to 4A may be used.
The invention also extends to passivating compositions for zinc/manganese alloys which unexpectedly give a black passivate and improved corrosion resistance.
Thus according to this aspect of the present invention an aqueous composition for forming a black passivate on the surface of a zinc/manganese electrodeposit is characterised in that it comprises hexavalent chromium, one or more carboxylic acids and a copper sulphate and is free of silver ions. The hexavalent chromium may be provided by a mixture of CrO3 and concentrated sulphuric acid. e.g. it may contain 30 to 70 g/l, preferably 40 to 60 g/l e.g. about 50 g/l of CrO3 and 2 to 15 ml/l, preferably 5 to 10 m/l of 96% H2SO4.
The composition preferably contains 40 to 100 ml/l, preferably 50 to 70 ml/l, more preferably 60 to 80 ml/l of acetic acid as the carboxylic acid.
The composition preferably contains 10 to 25 g/l of copper sulphate. e.g. CuSO4. 5H2O preferably 14 to 20 g/l more preferably 15 to 18 g/l.
The invention also extends to a method of providing a zinc/manganese alloy electrodeposit with a black passivate which comprises treating the electrodeposit with a passivate composition as claimed herein. Preferably the zinc/manganese electrodeposit contains 14 to 20% by weight of manganese.
The invention also extends to a zinc/manganese electrodeposit especially one made in accordance with the present invention whenever provided with a black passivate finish by a method as claimed herein.